An airbag system includes a gas generating source (an inflator) and an air bag. The gas generating source generates gas when a large acceleration is detected. The airbag is deployed and inflated by the gas generated by the gas generating source. The airbag is formed by stitching pieces of base fabric, each piece cut out into a predetermined shape.
In an airbag for vehicle, in order to effectively restrain a rider, it is preferable that the center of the ceiling wall of the airbag become planar when the airbag is deployed and inflated, and also that the amount of extension of the airbag towards the rider's side be limited to a predetermined amount. For this purpose, the cutting shape of each piece of base fabric forming an airbag is carefully designed so that the airbag can be deployed into a shape with which a rider is effectively restrained.
However, it is difficult to deploy an airbag into a desired shape only with the tension of base fabric. For this reason, in a conventional airbag system, as disclosed in Japanese Patent Application Laid-open Publication No. 2005-8055, a proximal end portion and a distal end portion in the inner surface of an airbag is connected to each other with a tether (straining fabric) made of a band-shaped piece of fabric. Accordingly, the shape of the airbag is stabilized with a tension caused by the straining tether when the airbag is deployed and inflated.
In a four-wheeled vehicle, an airbag on the driver's seat side is housed in the center portion of a circular steering wheel. When the airbag is deployed, the back side of the deployed airbag is uniformly supported by the entire surface of the circular steering wheel. Accordingly, it is easy to stabilize the position and shape of the airbag when the airbag is deployed and inflated. In addition, an airbag on the passenger's seat side is also housed in a planar instrument panel. For this reason, when the airbag is deployed and inflated, the back side of the airbag is uniformly retained by the instrument panel.
On the other hand, a two-wheeled vehicle and a three-wheeled vehicle often do not have a circular steering wheel. In addition, in the two-wheeled vehicle and the three-wheeled vehicle, a portion behind a space in which an airbag is deployed is not planar. For these reasons, while it has been possible to stabilize the shape of a deployed and inflated airbag, it has been difficult to stabilize the position.
FIG. 5 is a top view showing an example of a tether structure of a conventional airbag. In an airbag 10, three tethers 31L, 31R and 31H are positioned in a shape of isosceles triangle in the top view of the airbag at the time when the airbag is deployed and inflated. The three tethers 31L, 31R and 31H are then attached to the inner side of the airbag 10 so that a vertex of the isosceles triangle is positioned on the front face side which faces a rider 2.
However, in the airbag 10 having such a tether structure, the following problem occurs. When the airbag 10 in the deployed and inflated state restrains the rider 2, the airbag 10 is compressed in the back and forth direction by receiving a forward momentum of the rider 2. At this time, however, the expansion of the airbag 10 in the right and left direction is limited especially by the lateral tether 31(H), as shown in FIG. 6. As a result, the expansion of the airbag 10 in the right and left direction is limited also on the back face side. Accordingly, since an area where the airbag 10 is brought into contact with the vehicle is not extended, it is difficult to stabilize the position of the airbag 10.
Incidentally, in a case where the shape and position of a deployed airbag is unstable, a countermeasure has been sometimes taken, in which the volume of an airbag is increased. However, it is difficult to increase the volume of an airbag in the case of a two-wheeled vehicle and a three-wheeled vehicle each having severe constraints with respect to a space in which an airbag is installed, and also with respect to increase in weight.
An object of the present invention is to solve the above described problem in the conventional technique. Specifically, an object of the present invention is to provide an airbag, whose position and shape can be properly maintained without increase in the volume when the airbag is deployed and inflated, to a two-wheeled vehicle and a three-wheeled vehicle in which it is often difficult to house an airbag in the center portion of a circular steering wheel.